1. Field of the Invention
The present invention relates to a pattern forming method that utilizes a positive resist composition usable in a lithography process for manufacturing semiconductors, such as ICs, and circuit boards for liquid crystal displays and thermal heads, and other photofabrication processes. More specifically, the invention is concerned with a pattern forming method that utilizes a specified positive resist composition suitable for exposure performed with a projection exposure apparatus for immersion lithography using as a light source far ultraviolet light with wavelengths of 300 nm or below.
2. Description of the Related Art
As the demand for finer semiconductor devices has grown, efforts have been moving ahead to develop exposure light sources having the shorter wavelengths and projection lenses having the higher numerical apertures (higher NAs). Up to the present, steppers using as light sources ArF excimer laser with a wavelength of 193 nm and having NA of 0.84 have been developed. As generally well known, the resolution and the focal depth of these machines can be given by the following expressions;(Resolution)=k1·(λ/NA)(Focal depth)=±k2·λ/NA2 where λ is the wavelength of an exposure light source, NA is the numerical aperture of a projection lens, and k1 and k2 are coefficients concerning a process.
Although steppers using as light sources F2 excimer laser with a wavelength of 157 nm are under study with the intention of achieving higher resolution by further moving the exposure light sources to shorter wavelengths, it is very difficult to stabilize production costs and qualities of apparatus and materials since lens materials usable in exposure apparatus and materials usable in resist for ensuring exposure at shorter wavelengths are confined within very narrow limits, and there is a possibility of failing to bring exposure apparatus and resist having sufficient performance and stability to perfection within a required period.
As a technique of enhancing the resolution in an optical microscope, the method of filling the space between a projection lens and a test specimen with a liquid having a high refractive index (hereinafter referred to as an immersion liquid), or the so-called immersion method, has hitherto been known.
This “immersion effect” can be explained as follows. In immersion lithography, the foregoing resolution and focal depth can be given by the following expressions;(Resolution)=k1·(λ0/n)/NA0 (Focal depth)=±k2·(λ0/n)/NA02 where λθis the wavelength of an exposure light in the air, n is the refractive index of an immersion liquid relative to the air and NA0 is equal to sin θ when the convergent half angle of incident rays is represented by θ. That is to say, the effect of immersion is equivalent to the use of exposure light with a 1/n wavelength. In other words, application of the immersion method to a projection optical system having the same NA value can multiply the focal depth by a factor of n. This technique is effective on all shapes of patterns, and besides, it can be used in combination with super-resolution techniques under study at present, such as a phase-shift method and an off-axis illumination method.
Examples of apparatus in which this effect is applied to transfer of fine circuit patterns for semiconductor devices are presented in JP-A-57-153433 and JP-A-7-220990.
Recent progress of immersion lithography is reported in Proceedings of International Society for Optical Engineering (Proc. SPIE), vol. 4688, p. 11 (2002), J. Vac. Sci. Technol. B, 17 (1999), Proceedings of International Society for Optical Engineering (Proc. SPIE), vol. 3999, p. 2 (2000), and WO 2004/077158. When ArF excimer laser is used as a light source, it is thought that pure water (having a refractive index of 1.44 at 193 nm) is the most promising immersion liquid in terms of not only handling safety but also transmittance and refractive index at 193 nm. When F2 excimer laser is used as a light source, on the other hand, fluorine-containing solvents have been examined with an eye to balance between transmittance and refractive index at 157 nm, but no immersion liquid satisfactory in terms of environmental safety and refractive index has been found yet. From the viewpoints of the level of immersion effect and maturity of resist, it is thought that ArF steppers are earliest exposure apparatus equipped with immersion lithography.
From resists for KrF excimer laser (248 nm) onward, the image forming method referred to as chemical amplification has been adopted as a method of patterning resists for the purpose of supplementing sensitivity drops resulting from light absorption by the resists. To illustrate the image forming method by taking the case of positive-working chemical amplification, images are formed in a process that exposure is performed to generate an acid through decomposition of an acid generator in exposed areas, and conversion of alkali-insoluble groups into an alkali-soluble groups by utilizing the generated acid as a reaction catalyst is caused by bake after exposure (PEB: Post Exposure Bake) to render the exposed areas removable with an alkaline developer.
Since application of immersion lithography to a chemical amplification resist brings the resist layer into contact with an immersion liquid at the time of exposure, it is indicated that the resist layer quality is altered and ingredients having adverse effects on the immersion liquid are oozed from the resist layer. In WO 2004/068242, cases are described where the resists tailored to ArF exposure suffer changes in resist performance by immersion in water before and after the exposure, and it is pointed out that such a phenomenon is a problem in immersion lithography.
Use of chemical amplification resist in immersion lithography according to the foregoing general resist process in particular requires further improvements in development defects appearing after development.